The game for who will rule eastern Syria after the Islamic State (or ISIS) is on. On May 18, the United States destroyed a military convoy[2] allied with Syrian President Bashar al-Assad after it ignored repeated warnings to stop its advance on al-Tanf, a U.S. and British special operations base on the Syrian-Jordanian border. The base is covered under the October 2015 U.S.-Russian deconfliction agreement, which established a hotline for avoiding direct military confrontation between Russian- and U.S.-backed forces in Syria. This came a few days after the Russian base at Khmeimim[3] had declared that its air force, along with Iranian military advisers, would support Assad’s troops in their attempt to push east, clearing the road from Damascus to Baghdad and preventing the formation of a U.S.-supported buffer zone in eastern Syria. All of this followed Washington’s announcement on May 9 that it would provide heavier weapons[4] to the Kurdish factions of the Syrian Democratic Forces (SDF) to help them take ISIS’ capital, Raqqa—enraging U.S. ally Turkey.

Such actions are nominally about fighting ISIS. But the various actors in Syria are increasingly thinking about what comes next[5], with an eye toward the weaknesses of the other side. U.S.-Russian deconfliction agreements in Syria will likely soon be put to the test, increasing the likelihood for accidents at best. There is now a greater risk that the United States and its allies will be brought into direct military confrontation not only with Assad but with his Russian and Iranian backers—a risk that was previously tempered by the need of both factions to fight ISIS. To ensure that the risk of military confrontation remains manageable, the United States and Russia should agree on the parameters of a de-escalation zone in southern Syria that keeps the focus on ISIS and contains Iran’s ambitions for a land bridge through Syria to the Mediterranean. As things are going, however, this is unlikely to occur anytime soon, unless the Assad regime’s weaknesses are laid bare for its Russian and Iranian backers.

THE GREAT GAME

According to media reports, the May 18 U.S. strike on pro-Assad forces came after the column, made up of government troops and Shiite militiamen, refused instructions to turn around and ignored warning shots from U.S. aircraft. Although such aggressive moves by regime forces seemed to come from nowhere, bellicose messages had been emerging from the Russian air base at Khmeimim all week. These were noteworthy for their explicit intent and the degree to which they promised Russian cooperation with Iran. For instance, one message from the base’s Telegram feed stated:

The decision by Russia and Iran to encourage the Syrian government to push east comes in the wake of two major moves by the United States. The first was a missile strike against the Assad regime in April in response to its alleged use of sarin nerve gas[6]—a violation of international law (the Chemical Weapons Convention, which Syria joined in 2013 as part of Security Council Resolution 2118) as well as of a 2013 U.S.-Russian agreement[7] to eliminate Syria’s chemical weapons stockpiles. The second was Washington’s recent decision to ramp up support for the Kurdish-led SDF in its effort to liberate Raqqa.

Together, these moves signaled a deepening U.S. involvement in the Syrian war. The Americans’ previous plan for fighting ISIS in Syria, developed under former President Barack Obama, was based on support for the Kurdish People’s Protection Units[8] (YPG), the armed wing of the Democratic Union Party, a Syrian offshoot of the Turkish Kurdistan Workers Party (PKK). The PKK is officially considered a terrorist organization by the U.S. State Department and is the archenemy of Washington’s NATO ally Turkey. To square this circle, the United States created an umbrella organization around the YPG—the SDF—that encouraged non-Kurdish groups to help the Kurds fight ISIS in eastern Syria and the Euphrates River valley in return for U.S. support. The hope was that as the SDF took over more territory, it would gain support from the Kurds’ Sunni Arab rivals, who make up the majority of the population in eastern Syria and in the Euphrates valley in particular. The valley is the heartland of ISIS and its precursor, al Qaeda, and holding it is key not only to defeating the organization but, most importantly, to ensuring that it does not come back stronger in the future.

Militarily, the plan has been a success. The SDF has significantly reduced ISIS’ territorial holdings, surrounded Raqqa, and gained the admiration of U.S. military advisers. Politically, however, the SDF remains dominated by the Kurds. And although some Arab fighters have joined the SDF, to date they have mostly been either Arab Christians and other minorities or else Sunni Bedouin tribesmen, who are rivals of the settled tribes of the Euphrates Valley. Unless Washington can convince the YPG to give up its majority shareholding in the SDF and share power with the settled tribes, the Kurds’ ability to hold Raqqa and the rest of the Euphrates Valley seems like a long shot.[9]

Russia and Iran recognize that given the SDF’s limited ability to hold territory, the collapse of ISIS in eastern Syria presents an opportunity for the Assad regime to recapture lost ground. Over the past few months, Moscow and Tehran have thus been supporting the efforts of the government’s new Fifth Corps, an amalgam of pro-Assad militias with Russian and Iranian support, to push east from Aleppo to Manbij in north-central Syria—thereby cutting into Turkey’s buffer zone north of Aleppo—and then press south and east along the west bank of the Euphrates toward Raqqa. This move gives Russia multiple options: it can present the regime to the Sunni Arabs of the Euphrates Valley as a viable alternative to the SDF while leaving open the possibility of supporting the YPG in the event of greater U.S. cooperation with YPG rival Turkey. The move also enables Russia to play the role of spoiler to the United States’ plans in the region.

GETTING IN THE ZONE

The Russian-Iranian plan is ambitious, but it retains a critical flaw: the Assad regime’s depleted manpower means that it cannot conquer one piece of territory without exposing itself somewhere else. For instance, as the Fifth Corps pressed east over the past few months, the government began rapidly losing territory north of Hama, raising fears it would lose the city. Although Assad had repeatedly[10] used chlorine gas since the 2013 deal, it was only after these recent losses that his forces turned to the far more deadly sarin. This in turn prompted the U.S. missile strike on Assad’s Shayrat airfield, which destroyed around one-fifth of the Syrian air force. But it also sent a message: the United States will not allow the Assad regime to gas its way out of the conflict with Russian and Iranian help.

Realizing the regime doesn’t have the resources to fight on multiple fronts, Moscow and Tehran proposed in early May the creation of several “de-escalation zones,” or areas that the regime would not attack, in exchange for Russian and Iranian forces acting as “guarantors” that would monitor ceasefire violations. Such an arrangement would have allowed Russia and Iran to accept a de facto partition of the country without ceding control of the zones to neighboring countries, particularly Jordan and Turkey, supporting the opposition in those areas. From a U.S. perspective, the area with the greatest potential (and highest priority) for a prospective de-escalation zone is southwest Syria, including Deraa and the area abutting the Golan Heights. There, the armed opposition is more moderate—and, to date, more manageable—than in many other areas, and the Iranians’ presence is small enough that their ability to act as spoilers may be limited. As a result, southwest Syria is an area where a deconfliction agreement backed by U.S. and Russian security guarantees has a more realistic chance of succeeding.

Securing the south would not only protect U.S. allies Jordan and Israel from jihadists and the Assad regime, it would also provide a foothold for future anti-ISIS military operations to move east into the Euphrates Valley. Southern Syria’s Sunni Arabs, supported from Jordan, could also provide a possible alternative, or complement, to the SDF.

Russia’s announcement of joint operations with Iran to help Assad push toward the Euphrates also signaled that, as the Americans long suspected, Moscow intends to help Tehran secure a land bridge—a contiguous stretch of territory controlled by Iranian allies—from Iran through Iraq and Syria to Hezbollah-controlled areas in Lebanon, which would allow for the delivery of more and heavier weapons westward to Syria and Lebanon. Yet this is unlikely to convince other countries in the region, such as the Gulf states and Jordan, to stop supporting the opposition and may actually intensify the conflict. It could also provoke a response from U.S. President Donald Trump[11], whose administration seeks to counter Iran’s expansionist ambitions in places far outside its traditional sphere of influence, including the wilds of eastern Syria.

LOCATION, LOCATION, LOCATION

The outcome of the Russian- and Iranian-supported thrust eastward remains unclear. Over the past few days, pro-regime forces and allied Shiite militias—reportedly flying Russian flags[12] at the head of their columns—have been capturing territory from ISIS in the Badiya, the area east of Damascus in southern Syria. Yet taking and holding areas closer to the Euphrates will require many more troops, highlighting the regime’s manpower deficiencies and putting the Russians’ and Iranians’ willingness to escalate to the test.

The best way for Washington to manage this situation is to let the regime expend its energy pushing out toward the Euphrates while preparing for the advance to stall, as it looks likely to do. This requires sticking with the U.S.-Russian deconfliction agreements to protect al-Tanf while strengthening opposition groups in the region, many of which include Sunni Arabs and are therefore more politically acceptable to the locals. Initial indications[13] from Syrian and Russian sources are that Assad intends to take the ISIS stronghold of Deir ez-Zor in the central Euphrates Valley, giving the United States time to bolster opposition forces south toward Abu Kamal on the Iraqi border. Such an initiative would give the United States better options in southern Syria, help key U.S. allies Jordan and Israel, and temper Iran’s plans to dominate the Baghdad–Damascus road. It would also give the United States much-needed Sunni Arab political support for—or an alternative to—the Kurdish-dominated SDF in eastern Syria.